Ion retardation, using "snake-cage polyelectrolytes", is discussed in an article in Industrial and Engineering Chemistry, Vol. 49, No. 11, November 1957 (pp 1812-1819), titled "Preparation and Use of Snake-Cage Polyelectrolytes" by Melvin J. Hatch, John A. Dillon, and Hugh B. Smith. It is disclosed there that a "snake-cage resin" is a cross-linked polymer system containing physically trapped linear polymer. For example, acrylic acid, when polymerized in situ within the reticules of a cross-linked copolymer of styrene and divinylbenzene (DVB), forms a "caged snake" of polyacrylic acid. Ordinarily polyacrylic acid is soluble in alkaline aqueous solutions, but, when entrapped within the styrene DVB copolymer bead, does not dissolve out of its "cage". The article discloses, e.g., the use of Dowex 1 ion exchange resin with entrapped polyacrylic acid as a resin which is more selective for NaCl than for NaOH.
Preparation of the Dowex 1 ion exchange resin is reported in the literature as comprising the polymerization of styrene containing divinylbenzene (DVB) as a crosslinker. The amount of crosslinking is about 8%.+-.0.5%. The cross-linked styrene-DVB copolymer is chloromethylated in a Friedel-Crafts condensation employing, e.g., as catalyst, anhydrous AlCl.sub.3, ZnCl.sub.2, or SnCl.sub.2. The chloromethyl groups (--CH.sub.2 Cl) attached to phenyl rings along the polymer structure are quaternized by reaction with a tertiary alkyl amine, trimethylamine, thus forming ion-exchange groups along the polymer chain which may be illustrated by the following empirical structure: ##STR1## The resin particles are substantially spherical, generally have a standard mesh size (wet) of 20-50, and have a density (wet with 43% moisture content) of about 44 lb./ft..sup.3. Ordinarily the total exchange capacity (Cl.sup.- form) is about 3.5 meq./mg. (dry basis) and about 1.33 meq./ml. (wet basis).
U.S. Pat. No. 3,041,292 by Melvin J. Hatch also discloses ion exchange resins having entrapped polyacrylic acid. U.S. Pat. Nos. 3,078,140; 3,205,184; and 3,332,890, which are all continuation-in-parts of U.S. Pat. No. 3,041,292, also contain information relevant to the present invention. Canadian Pat. No. 575,049 also contains relevant information. U.S. Pat. No. 2,606,098 (alkali metal hydroxide method) and U.S. Pat. No. 3,228,275 (countercurrent working procedure) are somewhat related to the present subject matter.
Of the ion retardation resin systems disclosed in the above references which may be useful in desalting caustic, it is the type exemplified by commercially-available Dowex 1 ion exchange resin (with polyacrylic acid entrapped therein) which is of pertinence in the present invention. The various Dowex ion exchange resins are registered tradenames of The Dow Chemical Company.
Dowex 1 ion exchange resin is a strongly basic anion exchanger formed by co-polymerizing styrene with divinylbenzene (DVB) as a crosslinker and featuring, as nuclear substituents on the polymer chain, trimethyl benzyl ammonium quaternary salts. The resin is commercially available in the Cl.sup.- or OH.sup.- form, but for the present invention, the Cl.sup.- form is preferable.
In preparing the pertinent polyacrylic acid snake-cage resins, the ion exchange resin is filled with monomeric acrylic acid, then the acrylic acid is polymerized in situ in the exchange resin. The polymerization may be initiated or catalyzed by the use of a free radical catalyst, a redox catalyst, and/or by increasing the temperature to overcome the inhibitors normally used in stored acrylic acid.
Whereas the art suggests the use of various amounts of acrylic acid polymer to be used with a given amount of quaternary ammonium groups, the teachings suggest that a stoichiometric balance between the carboxylic groups and the quaternary ammonium groups is preferred in order that there be no substantial excess of either one when the resin is to be employed in desalting caustic.